The first commercially successful refillable PET carbonated beverage container, developed by Continental PET Technologies, Inc. in the 1980's, followed years of failed development attempts by others. The requirements for such a container included resistance to:
caustic stress cracking (due to repeated exposure to caustic cleaning agents at elevated temperatures, followed by repeated exposure to elevated internal pressure upon product filling)
dimensional instability (due to alternating cycles of thermal shrinkage and expansion)
base inversion or creep
drop impact failure
caustic whitening.
Prior development attempts may have satisfied one condition, while aggravating another. Disposable container designs were wholly unacceptable. Still further, a technically acceptable bottle design was required which could be sold at a price competitive with reusable glass containers. Once an acceptable refillable plastic container was found, it quickly eclipsed sales of returnable glass containers due to its light weight and shatter resistance.
Various embodiments of Continental PET's refillable preform and container designs are disclosed in U.S. Pat. Nos. 4,725,464, 4,755,404, 5,066,528, 5,198,248, and numerous foreign counterpart patents around the world.
An additional problem has been found with some refillable PET containers, especially those used in warmer climates--namely cracking around the sprue region of the interior base surface--referred to hereinafter as "sprue cracking." The "sprue" is an extended gate at the tip of the preform; the sprue cracks have occurred in the container around this central gate. Sprue cracking may prevent thorough cleaning of the container (i.e., contaminants may infiltrate the cracks), and/or the cracks may grow and result in a loss of pressurization and leakage of the bottle contents, i.e. a complete bottle failure. In addition, sprue cracking is undesirable because it causes whitening or opacity which is aesthetically undesirable and it also interferes with contaminant inspection, i.e., the inspection machinery considers the whitened or opacified region a contaminant and rejects the bottle.
All commercially-available refillable PET carbonated beverage containers are believed to provide a depression on the inner surface of the container base at the center of the dome, known as a "dish." Many commercial designs also have an external gate (projection) on the outer surface of the preform tip, and an internal dimple (projection) on the inner surface of the preform tip. The reason for the dish, gate and dimple is to ensure centering of the preform in the blow mold base.
Accurate centering is very important with a refillable bottle because it ensures that the standing ring (chime) region of the container base is blown to a uniform thickness. Without such uniformity, there is a tendency for the base to fail due to creep or caustic stress cracking. Thus, to ensure centering the external preform gate is positioned in a receptor (chamber) in the center of the blow mold base, and a dish-shaped opening in the mold base surrounding the receptor serves to guide the gate into the receptor. The internal preform dimple fits within a receptor on the tip of the axial stretch rod, to further ensure centering of the preform in the blow mold base. Still further, in the known processes there is a deliberate pinching (compression) of the preform end cap against the blow mold base, i.e., application of a compressive force, in order to hold the preform firmly in position during blowing and thus ensure accurate centering.
Thus, while the preform end cap has been carefully designed to satisfy the centering requirements, there has now been discovered a problem with sprue cracking for which neither the source nor the solution is known.